As examples of prior art methods and arrangements for transferring a process liquid between washing and thickening device and subsequent treatment stages, a few practical cases from pulp and paper industry will be discussed in the following. However, already at this stage it should be understood that similar processes exist, not only in pulp and paper industry, but also in several other industries like applications, for instance, in biomass processes and in the manufacture of biofuel.
A good example of the problem areas in pulp and paper industry are various washing and thickening stages, or washing and thickening device between the digestion of wood chips in chemical pulping and the web formation at a pulp or paper mill. Such means are needed, for instance in so called brown stock washing and in various washing stages when delignifying and bleaching the pulp. In pulp and paper making industry various different washing, filtering and thickening device are used. A first example of such is a single or multistage drum washer from which pulp is discharged at a consistency of 10-18%. Usually the pulp is discharged from such a washer by means of a discharge screw into a stand pipe or a feed chute such that the pulp is diluted in the discharge screw down to a consistency of 8-15%, which is the consistency of the pulp transferred further by means of a centrifugal pump from the bottom part of the stand pipe. In other words, most of the washing and filtering means presently in use are able to raise the consistency of the pulp to such an area that the pulp has to be diluted so that pulp could be fed to the next process stage (including also web formation or storage tower) by means of a state-of-the-art centrifugal pump or a fluidizing centrifugal pump though also positive displacement pumps may be used.
The stand pipe or feed chute is typically an upright vessel having a height of 5-7 meters. When the height required by the dilution means, possibly extending to several meters, is taken into account, the height at which the washing and thickening device are to be installed, for ensuring a sufficient height for the stand pipe and dilution, easily exceeds 10 meters. It means that the washing and thickening device is arranged one or two storeys or floor levels higher than the pump used for discharging the stand pipe. In fact, this is the main reason why pulp mills e.g. in bleaching plants need to have a second floor, or possibly a third floor, too, above the ground floor. In other words, the washing and thickening device must have been raised and supported some 6 to 15 meters above the ground floor for ensuring trouble-free pumping of pulp from the washing stage to a subsequent process stage. One reason why such high stand pipes are needed is the fluctuation in the discharge or production rate of the washing and thickening device. Such fluctuation may originate not only from the uneven operation of the discharge/dilution screw/s but also from the fluctuations in the production rate of the earlier treatment or process devices upstream of the washing and thickening device. For the above reason the stand pipe is used as a buffer tank whereby the level of the pulp in the stand pipe is measured and maintained within certain desired limits, i.e. between the upper and lower limits of the surface level control range. It is also, however, ensured that both the pump at the bottom part of the stand pipe and the washing and thickening device upstream of the stand pipe function without disturbance. This means, in practice, that the pulp should have enough residence time (meaning height) in the stand pipe so that pulp surface could be easily maintained between the allowable upper and lower limits.
Another good example of washing and thickening device, or, more generally, process devices is a wash press from which the pulp is discharged at a consistency of 20-40%, sometimes even at a consistency of up to 50%. Typically the discharge screw of a wash press is at the top part of the wash press (several meters (structural height of the wash press) above the floor on which the wash press is installed) such that the discharge screw moves the thickened fibre suspension from the wash press to a dilution means arranged to the same level, floor or storey with the wash press. In the dilution means the consistency of the washed and/or thickened fibre suspension is decreased down to 8-15%, too. The dilution means discharge the pulp to a stand pipe similar to the one discussed already above.
The dilution of pulp or fibre suspension after the washing and thickening device may be performed by means of the discharge screw of the washing and thickening device, by means of a separate dilution screw or by means of a separate mechanical dilution apparatus or the dilution means may be one or more dilution liquid conduits or nozzles arranged, in practice, immediately after the discharge screw of the washing and thickening device or the dilution may be performed at the top of the stand pipe. Thus the pulp is, in all prior art applications, diluted in advance of its entrance on the pulp column in the stand pipe or feed chute.
A further prior art example relating to transferring pulp from a washing and thickening device to a subsequent process stage by means of first diluting the pulp to a lower consistency and then pumping the pulp is discussed in the following. In accordance with this example the thickened pulp is discharged from the thickening washing means at a high consistency to the stand pipe such that the pulp is diluted in the pumping consistency substantially simultaneously with its entrance in the stand pipe. However, the bottom part of the stand pipe is, for safety reasons, provided with additional liquid introduction means, which are used for injecting liquid, normally, water into the pulp. These introduction means are used in such a case that the pump is, for some reason (e.g. non-homogenous dilution), not able to pump the diluted pulp properly without the additional dilution. However, the purpose of feeding the additional dilution liquid to the bottom part of the stand pipe is to adjust the consistency of pulp momentarily to a lower value, i.e. of the order of 2 percentage units or less lower (for instance from 12% to 10%) for ensuring the continuous operation of the pump by reducing the head requirement or reducing flow resistances in the discharge piping. However, such a dilution means is not intended for continuous operation, and it is not constructed to homogenize the consistency of the fibre suspension at the bottom part of the stand pipe, i.e. the means introducing the dilution liquid are not arranged to spread the dilution liquid to the cross section of the stand pipe.
In the following various problems encountered in the above discussed prior art arrangements are discussed.
The building where the washing and thickening device are located requires one or two storeys above the ground floor for ensuring a sufficient height for the dilution of the pulp and the stand pipe with its surface level control arrangements.
The dilution performed immediately after the washing and thickening device is not necessarily even or uniform but a part of the thick pulp and a part of the dilution liquid may enter the stand pipe without substantial mixing whereby the final mixing of the pulp and dilution liquid is performed by the pump discharging pulp from the bottom of the stand pipe.
Due to the location of the washing and thickening device at a height of one or two storeys or floors a pump is normally needed for feeding pulp to the washing and thickening device.
Since the pulp is diluted at such an early phase that the pulp column or layer in the stand pipe is of diluted pulp it is clear that the height of the level control range has to be substantially high to maintain a required pulp volume in the stand pipe. Increasing the diameter of the stand pipe or feed chute cannot be considered a preferred way of maintaining the control volume the same even if the height of the control range is decreased, as increasing the diameter of the stand pipe changes rapidly the flow conditions in the stand pipe. The pulp in the stand pipe starts easily to channel, which means that a part of the pulp column in the stand pipe remains in place whereas the rest of the column flows rapidly along “a channel” down to the pump. The standing part of the pulp may start to dewater and to filter, i.e. to dry due to gravity. It may also start to decay. In both cases parts of the standing pulp may loosen from the pulp column, and be pumped further, where they possibly reduce dramatically the quality of the end product. Also, the loosened parts of the standing pulp have a consistency significantly higher than that of the pulp normally flowing to the pump, whereby the loosened parts, depending on their size, may make the consistency of the pumped pulp fluctuate significantly.